DINOSAUR EXTINCTION THEORY BACKED
CRATER IN IOWA SEEN AS POSSIBLE CRASH SITE

Author: David Chandler, Globe Staff

Date: Monday, December 16, 1985
Page: 8
Section: NATIONAL/FOREIGN

The scientists, G. Izett and C. L. Pillmore of the US Geological Survey office in Denver, say they have found evidence that supports a theory first proposed by physicist Luis Alvarez, Nobel prize recipient,and his son, geologist Walter Alvarez. The theory, published in 1980, holds that an asteroid or comet slammed into the Earth at the end of a geological period called the Cretaceous - the age of the dinosaurs.

This explosive impact sent so much dust and soot into the atmosphere, the theory says, that sunlight was blocked and temperatures plummeted. Plants withered, and animals either starved or froze to death. The dinosaurs died out, making room for mammals and eventually for man.

Although many details are still being debated, the theory has achieved wide acceptance, according to many scientists gathered here last week for the annual fall meeting of the American Geophysical Union. A special session devoted to the extinction theory produced several new pieces of evidence to support the idea that an extraterrestrial cause - showers of comets or asteroids - may have played a crucial role in the rise and fall of Earth's life forms.

One opposing theory, defended here by Dartmouth College geologist Charles Officer, holds that it was volcanoes rather than impacts that drove dust and gases into the atmosphere and killed off the din osaurs. But most of the research presented here seemed to support the Alvarez impact theory.

Izett and Pillmore said that they had found great numbers of tiny grains of quartz that were cracked by the force of an impact. The grains were scattered around the world in layers of clay that formed exactly 65 million years ago - the so-called Cretaceous-Tertiary boundary, when more than half of all living species on earth suddenly perished.

Pillmore says the great number of quartz grains, in addition to providing ''strong supporting evidence" for the Alvarez theory, also shows that the impact must have struck on land, where the rocks are rich in quartz, rather than in the quartz-poor ocean bottom, as many scientists had thought.

Pillmore and Izett also found that the grains grew larger in the central United States area, while those found elsewhere in the world are only half as large. They believe that this suggests the impact was somewhere within 1,800 miles of the sites they studied in Montana and New Mexico, and led them to investigate a large crater near Manson, Iowa as the possible relic of the impact.

Although geologists had thought the Iowa crater was much older, Pillmore and Izett used sophisticated new dating methods to find the age of the structure and determined that it was no more than 69 million years old, placing it in the right age bracket for the Alvarez impact.

The impact theory was further supported by Piet Hut, an astrophysicist at the Institute for Advanced Studies in Princeton, N.J., who said that his research indicates that showers of comet impacts, spread out over about a million years, must occur occasionally. It may be, he said, that a series of events rather than a single catastrophe can better explain the gradual pattern found in the extinction of species.

Hut said that to settle the question of whether extinctions were caused by a single impact or by a series of them will require more detailed study of the fossil record. "We may have to look down in the rocks rather than up in the sky to find out about our solar system," he said.

Erle Kauffman of the University of Colorado has done just such a detailed study of fossil evidence, and says he has demonstrated that the fossil record shows that several mass extinction events, in which a large fraction of species were killed off, occurred in a series of steps rather than all at once. This pattern, he said, fits exactly what would be expected from the kind of comet showers predicted by Hut.

Ronald Prinn, a geologist at the Massachusetts Institute of Technology presented evidence that a comet impact would produce such extreme worldwide acid rain that this effect in itself might be enough to account for the extinctions, apart from the cold and darkness of the Alvarez theory. His calculations show that the passage of a comet through the atmosphere before impact would be enough to cause worldwide rainfall of pure nitric acid for at least a year.

Officer, who believes extinctions were caused by volcanoes rather than impacts, argued that the selective pattern of extinction - dinosaurs died, but mammals and birds didn't -is more easily explained by his theory.

But Officer conceded that he had no explanation of what could cause the massive episodes of volcanic eruptions that his theory requires, and that he has no evidence that volcanism on such a large scale ever has occurred.

On the other hand, geologist Paul Weissman of the National Aeronautics and Space Administration's Jet Propulsion Laboratory said it is clear from present knowledge of solar system dynamics that asteroid and comet impacts do occur, and so the kind of effects predicted by the Alvarez theory must also occur. ''As sure as the sun will rise tomorrow, these events do occur," he said. ''I'd like to see someone explain how these events could occur and not produce any effects."

chandl;12/14,11:49 CORCOR;12/17,13:29 DINOSA16